With the deployment of 3G wireless communication networks, the demand for mobile data service has increased dramatically in recent years. To meet the demand, operators are continuously improving network capacity and coverage. Introducing small cells is an efficient way of improving the capacity and coverage and is becoming the preferred choice of operators. Typically, small cells are deployed at hot spots to provide additional capacity/throughput or at areas with poor signal coverage to provide coverage extension. In addition, due to the lower transmit (Tx) power in small cells, both the energy consumption and electromagnetic pollution of the User Equipments (UEs) and the base station (BS) are reduced.
As compared with a macro cell, the overall path loss between the UEs and the BS in a small cell is lower. For example, as a measure of the path loss, the BS to UE minimum coupling loss (MCL) may be 53 dB for a micro cell, 45 dB for a pico cell, and even 30-40 dB for a femtocell. Thanks to the low path loss, a UE in the small cell may transmit with relatively lower power than that in a macro cell, while guaranteeing a good uplink (UL) signal quality to support desired data rate and Quality of Service (QoS). However, sometimes the UL signal quality may become excessively good and cause a problem. FIG. 1 illustrates a schematic small cell. In the small cell, both a UE1 and a UE2 are served by a Node B 110. The UE1 is located at the edge of the cell and the UE2 is located close to the Node B 110. In a practical communication network, a UE may not decrease its Tx power below its minimum Tx power limit. It is possible that for one or more UE, e.g. UE1, which are located very close to the Node B 110, the UL signal quality is still excessively good even if its Tx power has reached the minimum Tx power. In other words, the Tx power of the UE is more than needed. This is the so-called saturation problem. Such a problem may desensitize the Node B receiver and result in several negative impacts on UL. For example, it may lead to excessive UL interference to other UEs, especially to the UEs in the same cell, and in turn lead to waste of UE Tx power for both the UEs limited by the minimum Tx power and other UEs. Furthermore, the excessively good UL signal quality leads to a conservative use of the allocated load, which means that the other co-cell UEs will have less load headroom available. The decreased UL performance and the inefficient usage of UE energy caused by the saturation problem are undesirable.
A solution is proposed in “Uplink Interference Management for HSPA+ and 1×EVDO Femtocells”, by Yan Zhou, Mehmet Yavuz and Sanjiv Nanda, GLOBECOM'09 Proceedings of the 28th IEEE conference on Global telecommunications, Pages 4626-4632. In the solution, the Node B receiver is desensitized by attenuating the signal at the Node B receiver on purpose, which leads to a higher noise figure. The attenuation pulls the UEs with low coupling loss to a power controllable range and thus avoids transmitting at the minimum Tx power.
Although desensitizing the Node B receiver by attenuating the signal at the Node B receiver may avoid the UEs with low coupling loss transmitting at minimum Tx power, it does not really solve the problem. With desensitization, the UEs that originally transmit at minimum Tx power have to transmit at higher Tx power to achieve the same UL quality and data rate. As a result, the generated interference after attenuation and the consumed load headroom do not really decrease. The UEs that do not have the saturation problem will also need to increase their Tx power for achieving the same data rate due to desensitization, i.e. the problem of wasting UE Tx power is not mitigated but further deteriorated in the sense that more Tx power is needed to support the same data rate. Moreover, the increased UE Tx power will lead to an increased interference and consequently a decreased UL performance.